DEFORMATION OF THE CASCADIA ACCRETIONARY PRISM DRIVEN BY MEGATHRUST EARTHQUAKES

An accretionary prism typically consists of an outer wedge and an inner wedge, with the former featuring much more active deformation and a steeper surface slope. According to the dynamic Coulomb wedge theory, the inner wedge overlies the megathrust seismogenic zone which experiences stress drop during interplate earthquakes, but the outer wedges overlies the shallower aseismic (rate-strengthening) part of the megathrust which experiences stress increase during earthquakes. During most of the interseismic period when the seismogenic zone is locked, the shallow zone tends to relax, such that the outer wedge is in a stable state. It is the stress increase during earthquakes that drives the outer wedge into a state of compressive failure. Therefore, permanent deformation of the outer wedge takes place episodically during and shortly after great earthquakes. The structural transition between the inner and outer wedges may or may not be well defined, but the stress transition is always gradational such that the inner wedge may also experience limited compressive deformation. The stress regime and structure are also affected by other processes such as basal erosion and underplating.

The dynamic Coulomb wedge model readily applies to most of southern Cascadia. Northern Cascadia is complicated by the large sediment supply and rapid outboard growth of the prism during Pleistocene. The very frontal part of the prism often consists of a portion of very gentle slope, referred to as the proto-wedge. We reason that the proto-wedge is the result of high pore fluid pressure at the base of the recently accumulated thick sediment column on the incoming plate. When horizontal compression increases at the deformation front, probably as the stress increase caused by a megathrust earthquake is eventually transferred to this location via afterslip, the hydraulically weakened base of the incoming sediment section allows a decollement to develop, leading to a seaward jump of the deformation front. Between the new and old deformation fronts is the ephemeral proto-wedge that is in the process of being incorporated into the outer wedge. When wedge morphology is linked to the megathrust seismogenic zone, this complication should be recognized.